The invention relates to a circuit for measuring distances, of the general type disclosed in DE 42 25 968 A1 and corresponding U.S. Pat. No. 5,629,619. These prior patents disclose a circuit which is used to measure distances in a noncontacting manner by means of a measuring coil. The circuit is an inductive distance sensor, which is operated with relatively low frequencies in a range from 1 kHz to 10 kHz. To increase the accuracy of the measurement, it is necessary to determine the temperature influence on the measured value. In this known circuit, this occurs by means of a discrete circuit, which determines the dc voltage component with a circuit that is excited by ac voltages. The circuit has two inputs, which are activated in phase opposition by two input signals that are generated by a signal source. The operational amplifiers subsequent to the inputs operate with their resistors as voltage/current transformers, with the current being coupled into the measuring coil from both sides.
In the normal measuring operation, the circuit is activated with two ac voltages in phase opposition. To determine the temperature behavior, a dc offset voltage signal, i.e. dc component, is superposed upon the ac voltages. Since the circuit requires that the supplied current be equal at both ends of the measuring coil, different voltages adjust via the resistors, which are associated with the measuring coil, and additionally with respectively one operational amplifier circuit. These different voltages are caused by the offset of the ac voltages as well as the temperature-dependent components. The temperature-dependent output voltage, i.e. dc voltage, is determined with a further operational amplifier.
If one applies the superposition principle, one will see that the lower input of the circuit exhibits a low-pass behavior, and the upper input of the circuit a band-pass behavior. For ideal input signals in phase opposition, the total transfer function is thus a low-pass function, which is smoothed by a further capacitance. The low pass develops by the difference of a high-pass and an amplification path that is matched with it.
Since the offset in a first approximation is inversely proportionate to the temperature
                    U        =                              U            =                    ⁢                      K                                                            R                  0                                ⁡                                  (                                      1                    +                                          α                      ⁢                                                                                          ⁢                      T                                                        )                                            ′                                                          (        1        )            it is thus possible to determine the temperature, and to correct the temperature-caused measuring errors. However, these measurements are only seldom inserted into the normal measurements with the mere ac voltage input signals. During the measurement with the mere ac voltage input signals, it is also possible to determine a dc voltage component, which is used to determine and correct the temperature drift of the measuring coil.
The known circuit is problematic in particular to the extent that the occurring time constants of the filtration are very great, and that because of its setup, the known circuit is made relatively large and, therefore, unsuitable for an application in which only a very small space is available for the circuit.
It is therefore an object of the present invention to describe both a circuit and a method for measuring distances of the initially described kind, which enable a use of the circuit also when the space available for the circuit is small.